1. Field of the Invention
The present invention relates to a method of forming a microcrystalline silicon film, a method of fabricating a photovoltaic device using this method, and a photovoltaic device fabricated thereby, and particularly to a method of forming a microcrystalline silicon film using a plasma CVD method, a method of fabricating a photovoltaic device using this method, and a photovoltaic device fabricated thereby.
2. Related Background Art
Since the valence electron control of a thin amorphous silicon film became feasible in the latter half of 1970s, headway has come to be made in research and development for applying the thin amorphous silicon film to photovoltaic devices typified by a solar cell. As a process for fabricating a photovoltaic device having a thin amorphous silicon film or the like, there has heretofore been widely known a plasma CVD method utilizing radio frequency (RF) typified by 13.56 MHz.
According to the photovoltaic device having the thin amorphous silicon film or the like fabricated by the RF plasma CVD method, a comparatively good photoelectric conversion efficiency is achieved with a less material compared with bulk-monocrystalline or polycrystalline silicon. However, there is room for improvement in process speed. More specifically, the thickness of the amorphous silicon layer used in an active layer of the photovoltaic device is required to be about several thousands xc3x85. In order to obtain a good-quality amorphous silicon layer suitable for use in the photovoltaic device, however, it must be deposited at an extremely low rate. It has thus been difficult to reduce process cost.
By the way, in the plasma CVD method using 13.56 MHz, it has been confirmed that the quality of a thin film formed tends to be more rapidly deteriorated as the deposition rate of the film is increased, and so it is difficult to enhance throughput upon mass production.
A plasma CVD method using a microwave (MW) typified by 2.45 GHz has also been known as a method capable of forming a comparatively good-quality thin film even when the deposition rate of the thin film is comparatively high. For example, xe2x80x9ca-Si Solar Cell according to Microwave plasma CVD Methodxe2x80x9d, Kazufumi Azuma, Takeshi Watanabe, and Toshikazu Shimada, Preprints in 50th Science Lecture Meeting of Applied Physics Society, pp. 566 is mentioned as an example where an i- type layer is formed by the microwave plasma CVD method.
It has also been known that the use of the frequency of about 100 MHz within the VHF band is effective from the viewpoints of the provision of a high-quality thin amorphous silicon film and the high-speed formation of the film. For example, U.S. Pat. No. 4,933,203 describes the fact that xe2x80x9ca good-quality amorphous silicon film is obtained at a ratio, f/d of frequency, f (MHz) to an interelectrode distance, d (cm) of from 30 to 100 in a frequency range of from 25 to 150 MHzxe2x80x9d.
In this publication, the relationship between the frequency and the interelectrode distance is defined with respect to the production process of an amorphous silicon film. However, it describes neither the production process of a microcrystalline silicon film nor the deposition pressure and raw gas residence time in a frequency range higher than 150 MHz.
By the way, the thin film photovoltaic device using the thin amorphous silicon film generally has a pin junction structure, and the photoelectric conversion thereof is mainly conducted in an i-type layer. Many attempts to microcrystallize a p-type layer and/or an n- type layer in order to improve junction characteristics have been made to date. For example, Japanese Patent Application Laid-Open No. 57-187971 discloses a method of improving output current and output voltage by forming an i-type layer with amorphous silicon and forming at least a layer of a p-type layer and an n-type layer, which is situated on the side struck by light, with microcrystalline silicon having an average grain size of at most 100 xc3x85.
However, at the present time, in any forming process, a phenomenon (the so-called Staebler-Wronski effect) that the defect density of an i-type layer is increased upon exposure to light to cause the reduction of photoelectric conversion efficiency has become a great problem in practical use in pin-type solar cells using amorphous silicon for the i-type layer.
In recent years, it has been attempted to use i-type microcrystalline silicon for a photoelectric conversion layer of an amorphous silicon type thin film photovoltaic device. For example, in 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996, a group of Shah et al. in Neuchatel University has reported a pin-type microcrystalline silicon solar cell having a photoelectric conversion efficiency of 7.7% without being attended with deterioration by light, which was fabricated by using microcrystalline silicon for all the layers of a p-type layer, an i-type layer and an n-type layer.
A process for forming the microcrystalline silicon layers adopted by this group is basically the same as the constitution of the conventional RF plasma CVD method and does not use a high-temperature process above 500xc2x0 C. required for the formation of a thin crystalline silicon film such as a thin polycrystalline silicon film. It is also characterized in that the frequency of 110 MHz within the VHF band is adopted as plasma forming frequency.
As described above, the pin-type solar cell using the i-type microcrystalline silicon film formed by using the frequency within the VHF band has a great advantage in that the solar cell is not attended with deterioration by light though it may be fabricated by a low-temperature process.
According to the above-described report from the group of Shah et al. in Neuchatel University, the deposition rate of the i-type layer of microcrystalline silicon was 1.2 xc3x85/s, and the thickness thereof was 3.6 xcexcm. It is found by a simple calculation that the time required to form the i-type layer of microcrystalline silicon is as long as at least 8 hours. Therefore, its throughput is very small though it has a comparatively high conversion efficiency and is not attended with deterioration by light. As a result, it is difficult for such a process to reduce its production cost.
In order to realize the mass production of the pin- type solar cell using microcrystalline silicon for an i- type layer, it is essential to enhance the deposition rate of the i-type layer of microcrystalline silicon by leaps and bounds. However, pin-type solar cells using the microcrystalline silicon for the i-type layer and having a comparatively good photoelectric conversion efficiency have been comparatively lately fabricated, and so the technique for forming the i-type microcrystalline silicon layer at a high speed has been scarcely known under the existing circumstances.
For example, when a high-temperature process above 500xc2x0 C. is used, it is expected that energy for crystallization can be obtained as thermal energy from a substrate, and so the formation of a film at a high speed can be conducted with comparative ease. However, the use of the high-temperature process incurs a possibility that the deterioration of cell characteristics may occur due to mutual diffusion at a cell interface, and involves a problem that process cost is increased.
The present invention has been completed in view of the foregoing circumstances, and it is an object of the present invention to provide a method of forming a microcrystalline silicon film having excellent semiconductor characteristics.
Another object of the present invention is to provide a method of forming a microcrystalline silicon film, by which a microcrystalline silicon film having good characteristics can be formed even when the formation is conducted at a low temperature and a high deposition rate.
A further object of the present invention is to provide a method of forming a microcrystalline silicon film, by which a microcrystalline silicon film suitable for use in an i-type layer of a pin-type solar cell can be formed at a high rate of from 2 to several tens xc3x85/s even by a low-temperature process of, for example, 150 to 500xc2x0 C. without using the high-temperature process above 500xc2x0 C.
A still further object of the present invention is to provide a method of fabricating a photovoltaic device using any one of the forming methods of a microcrystalline silicon film by which the above objects can be achieved, and a photovoltaic device having a microcrystalline silicon film obtained by such a forming method as a component.
A yet still further object of the present invention is to provide a method of forming a microcrystalline silicon film with a raw gas containing at least a silicon compound by a high-frequency plasma CVD method, wherein the formation of the film is conducted such that the residence time xcfx84 (ms) of the raw gas in a film deposition chamber, which is defined as xcfx84 (ms)=78.9xc3x97Vxc3x97P/M in which V is a volume (cm3) of the deposition chamber, P is a deposition pressure (Torr) and M is a total flow rate (sccm) of the raw gas, satisfies xcfx84 less than 40.
A yet still further object of the present invention is to provide a method of fabricating a photovoltaic device utilizing the forming method described above, and a photovoltaic device having a microcrystalline silicon film obtained according to the above film forming method.
As described above, the present invention relates to novel methods of forming a microcrystalline silicon film, and photovoltaic devices having the microcrystalline silicon film formed by such a method as a photoelectric conversion layer. The constitution and action of the present invention will hereinafter be further described.
In the high-frequency plasma CVD method using a silicon compound such as silane as a raw gas, the crystallinity, defect density, photosensitivity and the like of a film deposited can be controlled by changing forming conditions (frequency of high frequency, high-frequency power, substrate temperature, deposition pressure, distance between a substrate and an electrode, flow rate of the raw gas, dilution rate of the raw gas, etc.).
As described above, the present inventor has gone ahead with the research and development of methods for forming a microcrystalline silicon film suitable for use in an i-type layer of a pin-type solar cell at a high rate of 2 to several tens xc3x85/s even by a low-temperature process of 150 to 500xc2x0 C.
As a result, it has been found that the range (xcfx84 less than 40) of the residence time of the raw gas exists as forming conditions for forming a good-quality microcrystalline silicon film, particularly, in a high deposition rate region of at least 5 xc3x85/s.
Here, the residence time, xcfx84 (ms) is defined in the following manner. Incidentally, deposition pressure, the total flow rate of the raw gas and the volume of a deposition chamber are regarded as P (Torr), M (sccm) and V (cm3), respectively.
When P1=760 (Torr), V1=M/60 (cm3/s), P2=P (Torr), V2=V (cm3)/xcfx84 (ms)xc3x97103 are substituted into the relational expression (Boyle""s law), P1V1=P2V2 between volume and pressure of a gas at the same temperature, xcfx84 (ms)=78.9xc3x97Vxc3x97P/M is derived.
In general, the decomposition of the raw gas is accelerated, and at the same time energy of ions and electrons present in plasma is also increased as frequency, f is increased under conditions that the pressure, P is fixed. It is therefore considered that a film deposited can receive energy from the ions and electrons in the plasma, thereby accelerating lattice relaxation to improve crystallinity.
When the frequency, f is continuously increased while keeping the distance, d between the substrate and the electrode constant, however, the energy applied to the deposited film from the plasma becomes too great, so that the defect density in the deposited film is increased, and in some cases the deposited film itself may not be formed due to etching effect. Therefore, the value of the distance, d (cm) between the substrate and the electrode, at which a good-quality film can be formed, increases with the increase in the frequency, f.
In U.S. Pat. No. 4,933,203 described above, the condition range of the ratio, f/d of the frequency, f and the distance, d (cm) between the substrate and the electrode for forming a good-quality film is defined. However, U.S. Pat. No. 4,933,203 describes the optimum conditions for forming good-quality amorphous silicon films and alloy films thereof, but refers to neither the formation of a microcrystalline silicon film nor the residence time.
According to the results of the research and development performed by the present inventor, a good-quality microcrystalline silicon film was able to be formed in the frequency, f ranging from 50 to 550 MHz in the formation of the microcrystalline silicon film by the high-frequency plasma CVD method using a silicon compound such as silane as a raw gas at a low temperature of 500xc2x0 C. or lower. However, the optimum conditions in the respective forming conditions thereof varied between the low-frequency side and the high-frequency side within the range of from 50 to 550 MHz.
It has however been found that when the relational expression, xcfx84 (ms)=78.9xc3x97Vxc3x97P/M of the residence time is used, the range of the conditions for forming a good quality microcrystalline silicon film can be defined over substantially the whole region of from 50 to 550 MHz, and the range falls within the range of xcfx84 less than 40. Incidentally, the residence time is desirably at least 0.5 ms. The reason for it is that if the residence time of the raw gas is too short, the application of energy to the raw gas is decreased to fail to sufficiently decompose the raw gas, so that the utilization efficiency of the raw gas is reduced to decrease the deposition rate of the film.
In order to obtain a good-quality microcrystalline silicon film at low cost, it may be preferred that the frequency of a high-frequency power source be 50 or more and does not exceed 550 MHz, the temperature of a substrate be 150xc2x0 C. or more and does not exceed 500xc2x0 C., the deposition pressure be 0.01 Torr or more and does not exceed 0.5 Torr, and the making power density of the high frequency be 0.001 or more and does not exceed 0.5 W/cm3.
It may be more preferred that in the frequency of the high-frequency power source being 50 MHz or more and not exceeding 200 MHz, the temperature of a substrate be 150 or more and doesnot exceed 500xc2x0 C., the deposition pressure be 0.1 Torr or more and does not exceed 0.5 Torr, and the making power density of the high frequency be 0.001 W/cm3 or more and does not exceed 0.2 W/cm3.
It may also be more preferred that in the frequency of the high-frequency power source being 200 MHz or more and not exceeding 550 MHz, the temperature of a substrate be 150xc2x0 C. or more and does not exceed 500xc2x0 C., the deposition pressure be 0.01 Torr or more and does not exceed 0.3 Torr, and the making power density of the high frequency be 0.01 W/cm3 or more and does not exceed 0.5 W/cm3.
Referring to the raw gas from the viewpoint of the formation of a good-quality film at a high speed, it may be preferred that the total flow rate of the raw gas comprising a silicon compound gas and a diluent gas be at least 500 sccm, and a ratio of the diluent gas to the silicon compound gas be at least 20.
In particular, it may be more preferred that the total flow rate of the raw gas be at least 1,000 sccm, and a ratio of the diluent gas to the silicon compound gas be at least 25. The ratio may be desirably at most 60. The reason for it is that in the case of dilution with H2 gas, the amount of H2 is increased, and so the quality of the film deposited may be deteriorated due to damage by etching effect, H ions and the like.
The methods of forming a crystalline silicon film according to the present invention feature that the characteristics of a film formed are scarcely deteriorated, particularly, even upon the formation of the film at a high deposition rate of at least 5 xc3x85/s.
In particular, the formation of a film at a deposition rate of at least 10 xc3x85/s is very preferred, because it is related to the great shortening of process time. The methods of forming a crystalline silicon film according to the present invention may provide films having characteristics comparable with those in a low deposition rate of about 1 to 2 xc3x85/s even when they are formed at a considerably high deposition rate of at least 10 xc3x85/s.
From the viewpoint of the enhancement of film quality of the resulting microcrystalline silicon film, it may also be preferred that a second high frequency, which does substantially not contribute to the decomposition of the raw gas, is superimposed on the plasma.
It may also be preferred that supply powers from first and second high-frequency power sources be controlled in such a manner that the self-bias voltages of the first and second high frequencies become respective fixed values, since the deposition rate and characteristic distribution of a film to be formed in the direction of its thickness can be improved.
According to the methods of forming a crystalline silicon film according to the present invention, as described above, good-quality microcrystalline silicon films may be formed at low cost and a high speed. The detailed forming mechanism of the microcrystalline silicon film is not known, but may be expected to be as follows.
In the range of the above-described forming conditions,
1) a film can be formed at a low temperature and a high speed, since the frequency range is higher than 13.56 MHz in the conventional RF plasma CVD method, and so the decomposition efficiency of the raw gas and the plasma electron temperature are high.
2) The amounts of radicals and ions arrived at the growth surface of a microcrystalline silicon film can be controlled by selecting the pressure, frequency, gas flow rate, supply power, etc., thereby changing the forming conditions to amorphous film forming conditions, microcrystalline film forming conditions or etching conditions.
In particular, it has been confirmed that when the microcrystalline film is formed in a high rate region of at least 5 xc3x85/s, it is effective to limit the residence time, xcfx84 (ms) of a raw gas, which is defined as xcfx84 (ms)=78.9xc3x97Vxc3x97P/M, in which V is a volume (cm3) of a deposition chamber, P is a deposition pressure (Torr), and M is a total flow rate (sccm) of the raw gas (silicon compound gas and diluent gas), to the range of xcfx84 less than 40.
It is considered that since the supply of the raw gas is sufficiently conducted within the range of xcfx84 less than 40, the forming conditions become conditions under which ions and radicals accelerating crystal growth are moderately incident while sufficiently supplying film forming species without exhausting gas species. It is also considered that when such a comparatively high flow rate as the total flow rate of the raw gas amounts to at least 500 sccm is used, an effect to prevent the raw gas in a discharge space from being excessively decomposed is brought about.
As a result, it is possible to prevent a malignant precursor, which forms the main cause of deterioration in the film quality of a film deposited, from increasing. It is considered that the prevention of increase in the malignant precursor is extremely important to the formation of a good-quality microcrystalline film, in particular, at a high rate of at least 5 xc3x85/s.
3) It is considered that when the ratio of the diluent gas to the silicon compound gas is controlled to at least 20, the effect of hydrogen abstraction in the film by a hydrogen radical is accelerated, whereby microcrystallization is accelerated.
4) The potential distribution of the plasma can be controlled by superimposing the second high frequency, which does substantially not contribute to the decomposition of the raw gas, on the plasma, whereby the ion species arrived at the growth surface of a film deposited can be changed to control the film quality of the deposited film.
5) Supply powers from the first and second high-frequency power sources are controlled in such a manner that the self-bias voltages of the first and second high frequencies become respective fixed values, whereby the power substantially applied to the plasma can be kept constant, and so the deposition rate and characteristic distribution of a film to be deposited in the direction of its thickness can be controlled with high precision.
A photovoltaic device having, as a photoelectric conversion layer, a microcrystalline silicon film formed by any one of the methods of forming a microcrystalline silicon film according to the present invention has a high conversion efficiency and high stability to light at low cost.
The photovoltaic device according to the present invention has excellent sensitivity to long wavelength and is hence suitable for use as a lower cell of a stacked type thin film photovoltaic device.
The photovoltaic device according to the present invention can be continuously formed on a substrate in the form of a band by a roll to roll system to reduce production cost to a great extent.